The invention and its background are perhaps best understood in terms of what are considered to be drawbacks and problems posed by existing spectrometers of the across-the-stack type which "look" across a stack containing a gas of interest.
Stack analyzers using infrared, ultraviolet or visible radiation involve a series of optical elements normally mounted from and above a rigid, generally planar, mounting plate called an optical bench to provide a sturdy and stable mounting alignment between light source, mirrors, lenses and detector. Each optical element must be carefully aligned and stably secured to achieve proper operation. To obtain the required alignment generally requires careful machining and assembly.
Stack gas analyzers that look across the stack require a hole in the stack wall on each side of the stack to permit the transmission of radiation from a source on one side of the stack to a detector on the other side of the stack. To prevent escape of stack gases when the stack is under positive pressure, an element called an air window is usually incorporated in the analyzer and receives clean air under adequate positive pressure so that the air is forced into the stack to protect the analyzer from hot and dirty stack gases. A motor-operated slide valve or a weighted shutter is built into the horizontal duct which connects the analyzer to the stack. The valve or shutter is operated to a closed position in the event of loss of pressure of the clean air so that dirty stack gases cannot pass through the duct to analyzer elements. These valves or shutters in the horizontal duct are sometimes unreliable because of large bearing surfaces, dirt deposits, and excessive friction forces involved in their operation.
It is known to be desirable to maintain a negative or low pressure in the space adjacent the lens or window of the analyzer that interfaces with the stack gases to avoid the accumulation of dirt on the lens or window. This has been done by locating the lens in the low pressure region of a venturi throat in the flow path of the clean air supply through the air window element to the stack. Typically the arrangement to do this has included an annular flow passage in which the clean air flow first flows in a direction away from the stack to the venturi throat and then centrally into the stack. This arrangement inherently extends the length of the light path, which is not desirable.
When the light source and detector assemblies are mounted on opposite sides of the stack they must be aligned so that the source beam hits the detector after its passage through the stack. This either requires that the two stack holes and the analyzer mounting flange be in perfect alignment when they are prepared, which is difficult as a practical matter, or else some means of angle adjustment must be provided. One typical scheme for obtaining the angle adjustment utilizes a series of bellville washers between the analyzer flange and the adaptor flange to which the analyzer flange is connected. The numerous washers and parts involved make the adjustment difficult.
The air window element through which clean air is passed into the stack is intended to prevent the buildup of deposits in the duct part of the window which penetrates the stack so that the light beam is unimpeded. The larger the hole in the stack wall, the larger the volume of air flow of clean air is required to obtain velocities of the clean air sufficient to carry potential deposits out of the duct into the stack. Nevertheless a reasonably large hole is required to accommodate beam adjustment and aiming. These two conflicting requirements are considered to prevent optimization of the air lens design with current arrangements.
To provide stable AOTF operation at all ambient temperatures encountered in its location on the stack, it is desirable to maintain the AOTF at a constant temperature if possible. It is desirable that this be achieved by heating the filter to at least the highest temperature the filter is likely to see in normal operation and maintaining it at that temperature at all times, but without impeding the passage of light through the filter.
The radiation source intensity should be maintained at a constant level of radiation, consume as little power as possible and conduct as little heat as possible to the AOTF. Most analyzers of which we are aware have a source that is exposed to the convective currents and dissipate energy by thermal loss so that source radiation varies. The source and lens are also usually exposed to the atmosphere and thus subject to contamination. Additionally, the AOTF material may be subject to oxidation.
The aim of this invention is to provide a spectrometer design having a radiation and source assembly construction and arrangement which provides solutions to these problems and drawbacks of the typical existing spectrometers.